This weekend, some 400 undocumented immigrants from Central America arrived at Greyhound Lines bus terminals in Tucson and Phoenix.

It was a staggering occurrence, but not nearly as remarkable as how they got there – courtesy of federal immigration officials.

The immigrants arrived at the bus stations after having been arrested and processed by federal authorities in the Rio Grande Valley in Texas, and flown to Arizona from there because the Border Patrol lacked the resources to deal with the growing flow of people crossing illegally into the Lone Star State, according to USA Today. Federal officials said all the detention facilities in Texas were full and that they couldn't take in any more migrants.

The practice, the newspaper reported, apparently has been underway for several months.

Groups of immigrants left in Arizona have included infants. Organizations that serve immigrants complain that federal authorities essentially are dumping people without food, water, or other necessities.

"It's not safe healthwise and we are concerned for their physical safety," said Cyndi Whitmore, a volunteer with an advocacy group that has been going to the bus terminal in Phoenix to help, USA Today reported.

Whitmore said that one day she found 50 women and children at a Greyhound station who had just been left there by U.S. Immigration and Customs Enforcement.

"Some of the kids were crying," Whitmore said. "Some were infants that weren't fully clothed. They didn't have diapers. They didn't have formula."

Organizations on the other side of the immigration debate are aghast over the appearance of large groups of undocumented immigrants, but because they say they federal government is being negligent about enforcing the law.

"This is a huge concern," said Ira Mehlman, a spokesman for the Federation for American Immigration Reform, a Washington, D.C.-based group that favors strict immigration enforcement.

"This is exactly the incentive for people to cross the border illegally," he said.

Volunteers who have been going to Greyhound to help immigrants left there say that they are seeing more women and children unlike before, when they saw mostly adult men and women.

Laurie Melrood, a volunteer family advocate in Tucson, said: "The conditions under which they are released are inadequate and inhumane.”

Volunteers have been trying to help by buying the immigrants bus tickets to join relatives in other places, and by bringing them food and water, among other things.

In response, volunteers in Phoenix and Tucson have been going to the bus stations to help the migrants make arrangements to buy bus tickets to travel to relatives in other cities. They also have been providing food, water and other necessities.

ICE officials say the immigrants who are released are families whom the Border Patrol arrested as they attempted to cross into the United States illegally. Immigration officials screen the arrested people, then order them to appear at an ICE office after about two weeks of their release.

The flow of undocumented immigrants has subsided in Arizona but risen in South Texas, USA Today said, adding that ICE is giving priority to removing criminals who are considered the biggest public safety threats.